Field of the Invention
The invention relates to a process for producing a product made of a contact material based on silver with iron oxide as the main active component. The invention also relates to a contact material and to a product made of such a contact material, in particular a contact piece for power-engineering switching equipment.
For a contact piece in power-engineering switching equipment, for example a power circuit breaker, as well as in DC, motor or auxiliary protection, contact materials based on silver containing particular active components have long been found to be suitable. Known active components that have a favorable effect on the switching properties of the contact material include both metals and metal oxides. Examples of metallic contact materials based on silver include silver/nickel (AgNi) and silver/iron (AgFe). Examples of oxide contact materials include, in particular, silver/iron oxide (AgFe.sub.2 O.sub.3).
The practical applicability of the contact material is determined by its electrical contact properties. The most important parameters in this regard are its life measured in the number of switching operations, which is determined by the erosion of the contact piece, the so-called overtemperature which results from the contact heating at the contact bridge and the terminals, susceptibility to welding and resistance to corrosion.
In order to measure the switching properties of a contact material, a testing switch described in Z.f. Werkstofftechnik/J. of Materials Technologies 7 (1976), pages 381 to 389 is customarily used, in which a contact piece made of the contact material is respectively placed.
Silver/nickel contact materials, in particular, have good switching properties but they have the disadvantage that nickel dust formed by friction in production or during operation, and nickel oxide formed as a product of the switching, can have harmful effects on the human body.
As a replacement material for the silver/nickel contact materials, Published, Non-Prosecuted German Patent Application DE 43 28 281 A discloses an oxide contact material based on silver, which contains iron oxide in proportions by mass of from 3% to 20% as the main active component, and at least one oxide of an element from the third subgroup of the Periodic Table in a proportion by mass of from 0.1% to 10% as a further active component. The use of yttrium oxide, in particular, as a further active component is in this respect regarded as particularly advantageous for the contact properties of such a contact material.
Likewise, Published, Non-Prosecuted German Patent Application DE 44 10 462 A describes a contact material based on silver with iron oxide as the main active component. In order to improve the contact properties, besides an oxide of an element in the third subgroup of the Periodic Table, in particular yttrium oxide, at least one metal oxide of an element in the 6th subgroup of the Periodic Table is also added. In particular, iron tungstate (FeWO.sub.4) reduces the contact heating and the susceptibility to welding, and increases the life of the contact material.
Furthermore, Published, Non-Prosecuted German Patent Application DE 41 17 311 A discloses a contact material based on silver, which contains iron oxide in proportions by mass of between 1% and 50% as the active components, and an oxide of another chemical element in proportions by mass of between 0.01% and 5%. It has in this case been found that the thermal behavior, in particular, of a silver/iron oxide contact material can be favorably affected by adding rhenium oxide, bismuth zirconate, boron oxide, zirconium oxide, molybdenum oxide or tungsten oxide.
As the production process for a contact material based on silver with iron oxide as the main active component, the Published, Non-Prosecuted German Patent Applications DE 43 28 281 A, DE 41 17 311 A and DE 44 10 462 A describe the mixing of suitable amounts of silver power, iron oxide powder and powder made up of other active components, and further processing the resultant powder mixture using powder metallurgy methods to form a semi-finished product or a shaped article. Commercially available powders are in this case exclusively used for the aforementioned powders.
There are essentially two different known powder metallurgy methods for producing a shaped article made of the contact material.
In the molding technique, the powder mixture is compressed by compression molding to give a molding which is processed further by sintering and, where appropriate, further compression to give a finished shaped article. For the production of a shaped article in the form of a contact piece, the molding may be further compression-molded with a layer of pure silver for secure connection of the contact piece to the support by hard soldering.
In the extrusion technique, the powder mixture is firstly compressed and/or sintered to give a porous preform or bar. The preform or bar is, where appropriate together with a layer of pure silver, extruded to give a rod from which the shaped articles are cut, and optionally subjected to a subsequent treatment.
A disadvantage of the silver/iron oxide contact materials produced by mixing silver powder with commercially available oxide powders is that it is difficult to alter their shape. Such contact materials are unsuitable for the production of rivets, in particular rivets with a large head/stem ratio. Since the head of the rivet is generally formed by compressive deformation of a shaped article in the form of a wire, one prerequisite for the material used is that its shape can be readily altered.